Preparations for tinting and dyeing hair are an important type of cosmetic agent. They can be used to nuance the natural hair color slightly or more heavily in accordance with the wishes of the particular person, to achieve a completely different hair color, or to cover undesired shades of color, such as shades of gray. Depending on the desired color and/or lastingness of the coloring, typical hair dyeing agents are formulated either on the basis of oxidation dyes or on the basis of direct dyes. Combinations of oxidation dyes and direct dyes are also frequently used to achieve specific nuances.
Dyeing agents based on oxidation dyes lead to brilliant and lasting shades of color. However, they require the use of strong oxidants, such as hydrogen peroxide solutions. Such dyes include oxidation dye intermediates: developer components and coupler components. The developer components form the actual dyes among each other or by coupling with one or more coupler components, under the influence of oxidants or atmospheric oxygen.
Dyeing agents based on direct dyes are often used for temporary coloring. Direct dyes are dye molecules that attach directly to the hair and do not require an oxidative process to form the color. Important representatives of this dye class are, for example, triphenylmethane dyes, azo dyes, anthraquinone dyes, or nitrobenzene dyes, each of which can bear cationic or anionic groups.
In the case of all these dyeing processes, it may happen that the coloring should be completely or partially reversed for various reasons. Partial removal of the coloring is useful, for example, if the dyeing result turns out darker than desired on the fibers. On the other hand, complete removal of the coloring is also desired in some cases. For example, it is conceivable that hair should be dyed or tinted in a certain nuance for a specific occasion and then the original color should be recovered after a few days.
Agents and methods for color removal are already known in the literature. A method for reversing colorings that is well known from the prior art is the oxidative decoloring of the dyed hair, for example by means of a typical bleaching agent. However, in this process, the fibers can be damaged by the use of strong oxidants.
Furthermore, reductive processes for color removal have also already been described. For example, European patent application EP 1300136 A2 discloses a method for treating hair in which the hair is dyed in a first step and reductively decolored in a second step. In this method, the reductive decoloring is performed by applying a formulation containing a dithionite salt and a surfactant. In WO 2008/055756 A2, the reductive decoloring of keratin fibers is performed by means of a mixture of a reductant and an absorbent.
If reductive decoloring agents are used, the decoloring occurs by the reduction of the dyes present on the keratin fibers or hair. As a result of the reduction, the dyes are generally converted into the reduced leuco forms thereof. In this process, the double bonds present in the dyes are reduced, the chromophoric system of the dyes is interrupted in this way, and the dye is converted into a colorless form.
A general problem of the reductive decoloring agents known from the prior art is that the dyed keratin fibers can initially be decolored by using the reductant but the color removal is not permanent. Particularly in the case of oxidatively dyed hair, in the case of which the coloring is produced on the hair by means of oxidation dye intermediates of the developer type and of the coupler type, colorings having very good fastness properties in some cases are obtained. When the reductive decoloring agent is applied, these dyes are then reductively converted into colorless compounds—which, however, still remain on the hair due to similarly good fastness properties.
After the reductant has been rinsed off, these reduced forms can then be gradually reoxidized under the influence of atmospheric oxygen. Because of this reoxidation, more or less pronounced recoloring occurs. This recoloring generally does not correspond to the shade of color in which the keratin fibers had been previously dyed, but rather can turn out unattractive in any manner and is therefore desired by the user of the decoloring agent all the less.
Therefore, is desirable to provide a decoloring agent for decoloring dyed keratin fibers that decolors dyed keratin fibers as completely as possible. The decoloring should be long-lasting, and the decolored keratin fibers should not suffer any recoloring, any nuance shift, or any post-darkening under the influence of atmospheric oxygen. The decoloring agent should exhibit good decoloring performance especially on keratin fibers that have been previously dyed by means of oxidative dyeing agents based on oxidation dye intermediates of the developer type and of the coupler type.
Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with this background of the invention.